1. Field Of the Invention
This invention relates generally to the field of adaptive digital control systems which maintain closed-loop performance despite changes in the open-loop system and the presence of applied bias forces, and more particularly to adaptive bias compensating digital control systems for magnetic disc storage devices.
2. Description of the Related Art
Magnetic disc storage device are used in data processing systems for storing relatively large amounts of information that can generally be accessed in milliseconds. Storage or retrieval of information from the disc is accomplished by a transducer read/write head that generates a signal based on the magnetic state of the disc proximate the head; or alternatively generates a magnetic field based on an applied signal. The high information storage density and the requirement for relatively fast data access imposes relatively severe constraints on read/write transducer head positioning system. Accurately positioning the transducer head is made even more difficult when the hardware components change due to age or environment. Magnetic disc storage devices incorporate a closed-loop control system which generally use current system state information in conjunction with a commanded position and/or velocity to move the head in some optimal or near-optimal manner. To some extent some control systems may attempt to adjust or compensate for certain changes in the hardware or environment. When a control system cannot adequately compensate for some system or environmental change, then these changes must be reduced to acceptable levels by some other means, such as by providing more precise tolerance hardware component. Positional accuracy, noise and disturbance rejection, robustness to hardware plant and environmental change, and the response time needed to achieve that commanded position are indicators of performance. There may generally be a tradeoff between each of these factors. Without using tight tolerance components, the performance of conventional controls systems applied to magnetic disc storage devices has been limited and does not provide the desired level of performance.
FIG. 1 is an illustration which shows an example of a disc storage device. Structurally, a typical storage device comprises a rotating magnetizable disc medium having at least one magnetizable surface 702, in the form of an assembly of one or more stacked platters 701, on which data is magnetically sensed and/or recorded in addressable sectors 703 located on circular data tracks 704. The disc assembly 706 is mounted on a drive spindle 707 in the storage device that rotates at a substantially constant speed. The storage device also includes one or more transducers or read/write heads 708, associated with each surface of the disc. The transducers are mounted on an arm 709 of a movable transducer or actuator arm carriage 710 which is linked to a servo actuator 711. A flexible cable 712 extends from the transducer head 708 along the actuator arm 709 with electrically connect the transducer head 708 to electrical read/write circuitry (not shown). The servo by way of the servo controller responds to a command from the interface to perform a track seek. The carriage is actuated in a controlled fashion to move all the mechanically coupled data heads in unison radially over the disc surfaces to position any one of the data heads over a selected track. Since all the transducer heads on the carriage move together, the device also includes head selection control circuitry that selects one of the read/write transducer heads 708 to perform a data transfer operation.
In a typical digital servo, the measurement that the digital controller reads from the heads consists of a coarse position indicator value such as the track crossing information, and a fine position indicator value such as the position error signal (PES) generated by demodulating pre-recorded servo information stored on the disc. The PES indicates the position error of the head away from the nearest track centerline. There are two primary types of servo drive systems: those incorporating a dedicated servo disc which stores only servo information but no user data, and sector servo systems wherein the servo code is recorded interspersed with user data. In a dedicated type servo drive system, the pre-recorded servo data is written on a dedicated disc platter surface. The track crossing information may be derived directly from the PES because of high PES sampling rate. However, in a sector servo, the servo data is written on the data discs interspersed between the read/write data segments, thus it is not possible to derive track crossing information from the PES because the system must recognize each track crossing and the heads could cross many tracks between PES samples.
In typical disc drives, the actuator is subject to bias components, generally at low frequency, which move the actuator away from the desired position during seek and track following modes. The bias could consist of but is not limited to: force from the flexible cable which is attached to the actuator coil, friction in the actuator bearings, windage due to air circulating inside the head disc assembly, gravitational force on the carriage assembly if the actuator is not perfectly balanced, and system electronic offsets which results in a constant current to be applied to the actuator with zero control effort.
The actuator and heads are electro-mechanical components and their parameters, such as torque constant, inertia, core width, and head gap, are subject to variation. In addition, other system parameters that can vary are AGC loop gain, servo demodulator gain, A/D converter gain, D/A converter gain, and servo power amplifier transconductance gain. The variation can be due to component tolerance, temperature, humidity, wear and aging.
In a conventional closed-loop system, it generally may not be possible to maintain the same nominal performance with the variations of system parameters that occur. One way to deal with this problem is to decrease the performance level so that a disc drive with worst-case components and environment can perform acceptably. Another way is to tighten component tolerance to achieve the desired performance, but this may increase costs substantially. Neither of these alternatives are particularly attractive. The control methods and apparatus according to the present invention overcomes these limitations.
Unlike other adaptive control techniques, the present invention will work with a system having an arbitrary bias force. The present invention identifies the plant gain constant while track following, whereas the conventional methods do it only while seeking. The adaptive algorithm described in this invention does not require or assume the system to be bias-free or have constant bias characteristics. The bias force can be arbitrary and slowly varying and its magnitude can be different in the forward and reverse seek directions. The calibration can be done one track at a time if necessary or desirable.
These techniques may require that the magnitude of the bias force be insensitive to seek direction, and provide an adaptive algorithm only for a second-order plant. See, for example, U.S. Pat. No. 4,697,127 issued to Stich et al., or U.S. Pat. No. 4,679,103 issued to Workman, and herein incorporated by reference. Both of these requirements do not apply to the present invention. The adaptive method of the current invention will work with any plant that can be adequately modeled as a linear system. Therefore, it will work with a plant of any order.
An objective of this invention is to provide an adaptive sampled data control system for a disc drive head positioning system.
It is also an object of this invention to provide an adaptive method which is independent of the control method used in the main control system and drive code.
It is a further object of this invention to provide an adaptive control system that generates a plant gain constant for each of the transducer read/write heads.
It is a further object of this invention to enable the firmware to measure the servo system's resonance frequencies assuming high enough sampling rate so that the resonance may be compensated for.
It is a further object of this invention to disclose an adaptive digital control system that has an estimator algorithm to estimate the position and velocity of the head, and the bias of the system.
It is a further objective of this invention to provide a method of calibration to deal with variations in components and environmental characteristics to maintain optimal performance without increasing cost.
It is a further object of the invention to provide a position measurement technique which extends the linear range of the position sensor across a plurality of servo tracks.
It is a further object of the invention to provide an apparatus that provides the digital adaptive control to a magnetic disc storage device or similar electro-magnetic plant.